The present invention relates to a position sensor which may be used, for example, as an electronic controller or a rotary switch, in particular for controlling a cooking hob.
Many rotary switches are used in the field of household goods in order to adjust washing programs in washing machines or temperature settings in electric cookers. Such rotary switches suffer, however from several drawbacks.
A significant drawback of currently existing rotary switches is that they are generally not free from wear and tear, and may suffer from deterioration in performance during their effective life. A further disadvantage is that they are generally complex to produce and complicated to contact with a control panel.
Selector switches may be incorporated on a control panel. Such switches have, however, a limited lifetime and they lack the flexibility of a rotary switch since the number of positions is limited by the geometric construction. Furthermore, as the number of positions increases, so the complexity of the switch also increases, increasing the cost. If sensors are employed which produce a sensor signal in response to a change of field caused by rotation of the rotary switch, this disadvantage can be overcome. Such sensors are, however, generally temperature sensitive, expensive and often show a hysterisis, which limits the number of switch positions. An example of such a field sensing switch would be one which incorporates a capacitive mechanism, wherein the capacitance is altered by the rotation of the selector. Such an arrangement is not suitable in many situations because of its sensitivity to dampness. Sensors which use the phenomenon of induction are resistant to dampness.
Inductive position sensors are known in the field of electric motors and the like for determining the angular position of the rotor. An example is described in U.S. Pat. No. 5,621,179. A rotary position sensor which employs planar coils is described in U.S. Pat. No. 4,507,638. In the arrangement described therein, a rotatable plate is interposed between drive and sensing coils. The rotatable plate is fabricated from a material which blocks a magnetic field induced in the drive coil from reaching or activating the sensing coil. To provide position sensitivity, the rotatable plate may have a cam-like shape. Alternatively, a plurality of drive and sense coils may be used having a distribution around the operating angle. Pulsed or A.C. signals are applied to the drive coils, which may be in the form of a planar coil on a printed circuit board.
A further position sensor is described in U.S. Pat. No. 5,239,288. The sensor described therein comprises a stator assembly and a rotor assembly. Each of the stator and rotor assemblies comprise a plurality of layers upon which one or more multi-turn coils are arranged. An A.C. signal is applied to one of the stator windings which results in a current being generated in the rotor windings. As a result, a signal which is dependent on the angular position of the rotor is generated in further coils of the stator.
In particular, the present invention provides a position sensor for determining a set position thereof, characterized in that the sensor comprises:
a time-varying magnetic field generator;
an electrically conductive element positioned such that said time-varying magnetic field generates eddy currents therein;
at least one pick-up for generating a signal in response to said time-varying magnetic field; and
a signal processor arranged to receive said signal and to determine the set position of the position sensor,
wherein the conductive element is movable relative to the at least one pick-up such that the signal generated by said pick-up has a magnitude which is dependent on the position of said conductive element, the position of the conductive element corresponding to the set position of the sensor.
The sensor may comprise at least two pick-ups, with signals from the two pick-ups being processed in combination to determine a sensor set position. Certain embodiments of the invention, in particular a linear position sensor require only a single pick-up for the sensor set position to be determined. The pick-ups are preferably in the form of an inductively coupled sensor loops having a combined projected area which is less than a projected area of a drive loop.
In a particular embodiment, the magnetic field is generated by a circular drive loop, which may be spiral in design and detected by two sensor loops which may each also be spiral, generally in the form of a sector. Several sensors may be multiplexed together, with a single processor determining the set position of each sensor. If desired, an isolation switch can be incorporated into the sensor to provide a positive disconnection, for safety reasons.